We recently demonstrated polypeptide polymorphism within the amino terminal region of MM, NN, and MN sialoglycoproteins of the human erythrocyte membrane, which is blood type specific. We now propose to extend these studies. Our approach includes examination of its cyanogen bromide fragments which we isolate in nearly quantitative yields from erythrocytes of single donors. We plan to 1) establish whether the polypeptide polymorphism extends to other regions of the molecule 2) to experimentally prove that the glycoproteins in the heterozygote (MN) are comprised of equimolar copies of the M and N glycoproteins by isolating each from the MN glycoproteins 3) to locate more precisely the locus of M and N antigenic determinants 4) to establish the detailed chemical structure of the asparagine-linked carbohydrate unit in sialoglycoprotein and determine the number and distribution of such units in the molecule.